Since automobile tires are manufactured using rubber compositions made from natural rubber and/or synthetic diene rubbers, degradation of such tires is accelerated at high oxygen or ozone concentrations or under ultraviolet rays, which may result in the formation of cracks. In order to suppress crack formation and growth in the presence of ozone, for example, additives such as antioxidants, e.g., N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (6PPD) or poly(2,2,4-trimethyl-1,2-)dihydroquinoline (TMDQ), or petroleum wax are used in rubber compositions.
The antioxidants and petroleum wax in rubber vulcanizates migrate (bloom) to the rubber surface of, for example, tires, thereby serving to protect the rubbers from ozone. Unfortunately, excessive blooming of the antioxidants and petroleum wax in a short period of time causes white discoloration. Moreover, the antioxidants oxidized by ozone cause brown discoloration, and similar excessive blooming of them intensifies brown discoloration. Furthermore, if the wax and the like bloomed on the tire surface form an uneven bloom layer (surface-protecting layer), diffuse reflection of light occurs, making the brown discoloration caused by the degraded antioxidants more noticeable. Thus, it has been difficult to improve ozone resistance while preventing discoloration.
Patent Literature 1 describes that the addition of a polyoxyethylene ether nonionic surfactant prevents deterioration of the appearance of tires. This technique still leaves room for improvement in terms of preventing discoloration and improving ozone resistance while maintaining or improving good elongation at break and good fuel economy.